Fuel processors in fuel cell systems convert hydrocarbon fuel, such as gasoline, into a rich hydrogen stream. Gasoline is reformed to the hydrogen stream through a series of reactions with steam and/or air. Using exothermic heat, steam is generated, superheated, and fed into the fuel processor. Operation of the integrated fuel processor requires effective balancing of the reaction chemistry. Balancing is accomplished by controlling reactor temperature and pressures, stream composition, and steam generation. The temperature of the heat exchanger affects steam generation.
Water Gas Shift (WGS) or shift reactors exhibit slow temperature dynamics with large time delays and are nonlinear over turndown. The amount of a reaction bed of the shift reactor that is used is directly proportional to the power level. For full power levels, reaction occurs throughout the shift reactor. For low power levels (such as idle conditions), only a small front section of the shift reactor is utilized. Under these low power conditions, the front and back ends of the shift reactor are typically at different temperatures. Consequently, active control of the temperature of the front-end does not adequately control the temperature of the back end. The lack of control cascades to the downstream reactors and ultimately impacts the generation of steam.
Currently an operator actively adjusts a desired temperature or setpoint of the front end of the reactor in lab environments. This may be acceptable on an experimental bench. For successful operation in a vehicle, however, the temperature of the front end must be adjusted automatically.
Referring now to FIG. 1, a conventional feedback controller 30 for a shift reactor 12 is shown. A front-end temperature sensor 14 senses a temperature of a front end 15 of the shift reactor 12. A pipe or conduit, an autothermal reactor, a partial oxidation reformer, or another shift reactor, which are generally identified at 16, may be located upstream from the shift reactor 12. A length of an exothermic section of the shift reactor 12 depends upon a power level of the shift reactor 12. For high power levels, the exothermic section may extend from the front end 15 to a back end 17 of the shift reactor 12. For lower power levels, the exothermic section may extend partially between the front end 15 and the back end 17. A reformate stream 18 is input to the front end 15 of the shift reactor 12. A back end temperature sensor 20 senses a temperature of the back end 17 of the shift reactor 12. A reformate stream 22 is output by a heat exchanger 24 to downstream reactors and vaporizers.
The front end temperature sensor 14 is connected to a feedback controller 30. The feedback controller 30 generates a flow signal to a water injector 32, which injects water into the front end 15 of the shift reactor 12. The water cools the front end 15 and provides temperature control. A temperature setpoint lookup table (LUT) 34 generates a desired front end temperature based on the fuel processor desired operating conditions, such as power level. The desired temperature is output from the lookup table 34 to the feedback controller 30. The feedback controller 30 outputs a water flow rate command to the injector 32.
The conventional feedback controller 30 measures and controls the temperature of the front end 15 to the desired temperature by metering the amount of water that is injected in the reformate stream 18. The injected water adjusts the front-end temperature quickly (on the order of tens of seconds and negligible time delay). Consequently the feedback controller 30 is capable of controlling the temperature of the front end 15 within a very narrow temperature range.
However, the conventional feedback controller 30 does not actively control the temperature of the back end 17 of the shift reactor 12. The temperature of the back end 17 can drift even when the temperature of the front end 15 is controlled. For example, factors such as power level, heat loss to ambient, variation in CO in the inlet reformate stream, catalyst degradation, low steam to carbon ratio, and other factors may cause the temperature of the back end to drift. Drifting of the back end 17 to a lower temperature will eventually cool the downstream low temperature shift or other downstream reactor and adversely impact steam generation by the PrOx vaporizer. A lack of temperature control of the back end 17 can also cause runaway in the fuel processor, which requires a shutdown.